As the design rule for a metal oxide semiconductor field effect transistor (MOSFET) gets reduced gradually, various problems such as short channel effect, drain induced barrier lowering (DIBL), and gate induced drain leakage (GIDL) occur. As a material for usage of channel to overcome these problems, compounds in III-V family and germanium (Ge) are rising in substitution for silicon. A compound semiconductor of III-V family is attracting attention as a channel material of n-MOSFET because of its high electron mobility, and Ge is attracting attention as a channel material of p-MOSFET because of its highest hole mobility. And in structural aspect, a partially depleted SOI structure and a fully depleted SOI structure based on silicon on insulator (SOI) substrate are in the limelight as structures to substitute conventional MOSFET of planar structure.
A Ge substrate may be used to use Ge as a channel material. A method of growing Ge on a silicon substrate is used since a Ge substrate is in very high price. However, direct growing Ge on a silicon substrate may cause a threading dislocation by a big difference of lattice constant between Si and Ge. Hence, a method is known that reduce the dislocation by forming silicon germanium (Si—Ge) layer with concentration escalation from Ge of low concentration to Ge of high concentration in a thickness of several micrometers on a silicon substrate and then form Ge of high concentration on the most upper portion. However, there exists a critical thickness in Si—Ge layer at which a dislocation does occur when the Si—Ge layer deposition is made on Si by Ge concentration. Therefore, The Si—Ge layer is formed thick to form Ge without final dislocation. So, formation of thick Si—Ge layer in this method cause a problem of high price despite a price merit compared to Ge substrate.
Meanwhile, forming a Si—Ge having Ge of low concentration at which a threading dislocation does not occur on a SOI substrate and getting it in oxidation at a certain temperature may cause Si atom to get a reaction with O atom to form a silicon dioxide (SiO2) layer with Ge concentration getting higher in a Si—Ge layer that gets thinner. Therefore, a germanium on insulator (GeOI) substrate may be formed having Ge concentration of 100% with sufficient oxidation. A process containing such a oxidation process is called as a germanium condensation process. A manufacturing process of GeOI substrate using the germanium condensation process has a merit of low cost and short process time.
However, in a conventional germanium condensation process, the process goes by implementing oxidation process and deposition of Si—Ge layer on a SOI substrate at a single temperature. Therefore, a problem of declination in concentration uniformity and surface roughness occurs. That is to say, a conventional condensation process is implemented at a temperature of generally higher than or equal to 1000° C. after deposition of a Si—Ge layer with Ge concentration of less than or equal to 30 at % on a SOI substrate. However, melting point gets lower as Ge concentration gets higher which cause the Si—Ge layer to get into liquid state, not solid, to make the concentration uniformity and surface roughness, etc. of the Si—Ge layer severely bad. And, conventional condensation method proceeds oxidation process after the deposition of the Si—Ge layer on the SOI substrate, and both of Si atom and Ge atom exist on the surface where the oxidation starts, which cause the oxygen to react with the Si atom in advance than the Ge atom. Therefore, uniform oxidation on entire surface of the substrate is not produced to make a problem of largely increased surface roughness.
As a method to solve such a problem, implementing oxidation process at two temperatures is proposed in JP2004-363199A. This technology may solve the surface roughness problem occurring during a condensation process. However, continuing condensation process to get higher Ge concentration may make uniformity of Ge concentration fall in a vertical direction of substrate. That is to say, the surface makes oxidation reaction with oxygen first, and Ge concentration gets gradually higher from the surface of the oxidation film. This phenomenon cause generation of stepwise concentration difference. When a difference of concentration occurs in stepwise way, physical characteristic and electrical characteristic may be changed by depth, which requires an even concentration distribution.